Cap,
how would the position of the CoG have any effect on the horizontal forces if you are flying in a straight line? There's no acceleration in any direction except the gravitational acceleration, which by definition is acting perpendicular to the horizontal.
If the moments don't add up, the angular acceleration will be about the CoG. However, the moments can be summed up about any point and will be the same regardless of the position of the CoG.
Often you use the CoG (if known) as the reference point for moment calculations. Since the weight is usually a factor, this means the moment arm from the reference point to the CoG is zero and the weight can be disregarded. The moment is the same regardless of reference point though.
Regards,
Fred